High-temperature flexible WSe2 photodetectors with ultrahigh photoresponsivity

The development of high-temperature photodetectors can be beneficial for numerous applications, such as aerospace engineering, military defence and harsh-environments robotics. However, current high-temperature photodetectors are characterized by low photoresponsivity (<10 A/W) due to the poor optical sensitivity of commonly used heat-resistant materials. Here, we report the realization of h-BN-encapsulated graphite/WSe2 photodetectors which can endure temperatures up to 700 °C in air (1000 °C in vacuum) and exhibit unconventional negative photoconductivity (NPC) at high temperatures. Operated in NPC mode, the devices show a photoresponsivity up to 2.2 × 106 A/W, which is ~5 orders of magnitude higher than that of state-of-the-art high-temperature photodetectors. Furthermore, our devices demonstrate good flexibility, making it highly adaptive to various shaped surfaces. Our approach can be extended to other 2D materials and may stimulate further developments of 2D optoelectronic devices operating in harsh environments.

Photodetectors are considered to be the core of modern communication components. As the development of aerospace, military, underground exploration, and harsh environment robotics, photodetector capable of operating at harsh environments are highly desired. Current high-temperature photodetectors are mainly based on wide-bandgap materials, such as SiC, GaN, or Ga 2 O 3 . Their optical sensing capability is barely satisfactory. A typical SiC photodetector can endure 550°C and its photoresponsivity (one of the most essential figures of merit for photodetector) is only 0.54 A/W 1 . GaN photodetector is able to operate in a wide temperature range from −196°C to 527°C 2 , but it demonstrates very low photoresponsivity of 0.02 A/W. So  There is a trend that photodetectors which can endure high temperature demonstrate very low photoresponsivity. The photoresponsivity of state-of-the-art hightemperature photodetectors is usually below 10 A/W. As such, weak light detection at high temperature is challenging. Furthermore, flexibility is an important development trend for next generation photodetectors. Flexible high-temperature photodetectors are desired in many applications, such as aeroengine blade with flexible (optical) sensors attached on its curved surface, and fully adaptive soft robot working in high-temperature environment with highly flexible sensors integrated on soft artificial muscle. However, high-temperature photodetectors reported so far are rigid. Limited by the poor thermal stability of traditional flexible materials 5 (polyethylene terephthalate (PET), polyimide (PI), polydimethylsiloxane (PDMS) and so forth), the temperatures that existing flexible optoelectronic devices can endure are below 300°C 6,7 . Consequently, highly-sensitive high-temperature photodetectors with good flexibility are highly desired.
Two-dimensional transition metal dichalcogenides (TMDs) have drawn a great deal of attention in optoelectronics field due to their fascinating physical properties 8,9 and facile preparation. They can reach monolayer or few layer by mechanical exfoliation 10 , liquid-phase exfoliation 11 , or chemical vapor deposition method 12,13 . Although graphene is the most well-investigated 2D material, TMDs have the advantages of higher sensitivity and lower dark current, and demonstrate remarkable optical sensing capability at room temperature. The photoresponsivity of monolayer MoS 2 photodetector reaches 880 A/ W 14 . Xie et al. reported photodetectors based on multilayer WSe 2 flake with photoresponsivity of 1.5 × 10 5 A/W 10 . Monolayer WSe 2 photodetector demonstrates higher photoresponsivity of 1.8 × 10 5 A/W 12 . Additionally, good mechanical properties (large fracture strain 15,16 ) make TMDs promising channel materials for flexible photodetector. However, TMD electronic/optoelectronic devices cannot survive at high-temperature. MoS 2 starts to oxidize and degrade at 300°C in air, resulting in many triangular etch pits on its surface [17][18][19] . The edge of WSe 2 flake begins to oxidize at 300°C in air 20 . It hinders the application of TMDs in aerospace and many other harsh environments. Importantly, it hinders the exploration of optoelectronic properties of TMDs at ultrahigh temperature.
Here, we developed WSe 2 photodetectors with h-BN (hexagonal boron nitride)/GF (graphite flake) heterostructure protection which can endure 700°C in air and 1000°C in vacuum. Unconventional negative photoconductivity (NPC) phenomenon appeared at high temperature. Operated in NPC mode, the device exhibited photoresponsivity of 2.2 × 10 6 A/W, which is~10 5 -fold higher than that of state-of-the-art high-temperature photodetectors, and even higher than that of existing WSe 2 photodetectors. The photodetector demonstrated good flexibility and realized in situ high-temperature optical sensing under bending state.

WSe 2 photodetector fabrication and characterization
We fabricated WSe 2 field-effect transistor (FET) as photodetector with h-BN encapsulation and GF source/drain electrodes (Fig. 1a) by mechanically stacking each atomic layer sequentially on freshly cleaved mica surface ( Supplementary Fig. 1) followed by photolithography, Pt deposition, and lift-off for top gate fabrication. The optical microscope image of a representative WSe 2 device is shown in Fig. 1c (Pt top gate is not shown). Mica substrate with thickness of~100 μm is highly flexible (bending radius <2 mm, Fig. 1b), high-temperature-resistant, and transparent. Additionally, compared with other flexible substrates, mica provides atomically flat terraces over large areas, so 2D materials can approach the limit of atomic flatness on mica surface and get rid of the microscopic corrugations which result in carrier scattering and degradation of electrical properties 21 . Multilayer WSe 2 flake was chosen as channel material of high-performance photodetector. Hexagonal boron nitride (h-BN) has been reported in previous work to be excellent oxygen-resistant coating 22 . We placed two h-BN flakes with thickness of~50 nm on top and bottom of WSe 2 , respectively as high-temperature encapsulation, where top h-BN also serves as gate dielectric ( Supplementary Fig. 2). The 2D material flakes were all characterized by Raman spectroscopy (Supplementary Fig. 3 and Fig. 2c). Sharp Raman peaks imply that the 2D materials studied in this work have nearly perfect lattice structures. Figure 1d is the highresolution transmission electron microscopy (HRTEM) cross-sectional image of h-BN/GF/WSe 2 /h-BN van der Waals heterostructures in the WSe 2 FET (Supplementary Fig. 4). The thickness of monolayer WSe 2 (~0.7 nm) is consistent with reported values 23,24 . It can be seen clearly that the 2D materials are atomically flat and no air gap is observed at interface, indicating good encapsulation, which is essential for hightemperature protection.

High-temperature-resistant capability
To investigate the high-temperature-resistant capability of our device, the WSe 2 FET was heated at 500°C, 600°C, and 700°C for 15 min sequentially in an open quartz furnace. The high-temperature experiments in this work were all carried out in air, unless otherwise noted. WSe 2 flake protected by h-BN showed negligible change after heating (Fig. 2b, Pt top gate is not shown). Raman spectra showed E 1 2g peak (~249 cm −1 ) and A 1g peak (~258 cm −1 ) corresponding to WSe 2 , whereas no WO 3 character peaks (~700 cm −1 or~810 cm −1 ) was observed [25][26][27] (Fig. 2c), implying that WSe 2 was not oxidized. After 700°C heating, flexible mica substrate was in good shape as well. Then the same device was heated at 750°C for 15 min. The WSe 2 channel still showed negligible change ( Supplementary Fig. 5), but mica substrate became brittle and less transparent. As control groups, bare WSe 2 and WSe 2 covered with 100 nm Al 2 O 3 (a widely used oxidation-resistant coating) by atomic layer deposition were heated at 500°C for 15 min. In both situations, the WSe 2 flakes were strongly oxidized and became almost transparent ( Supplementary Fig. 6) inasmuch as WO 3 is transparent under visible light. Raman spectrum shows prominent WO 3 character peaks around 700 cm −1 and 810 cm −1 (Fig. 2c). Therefore, bare WSe 2 has poor thermal stability, which is consistent with previous reports 20 . The h-BN is superior to Al 2 O 3 as high-temperature protection layer.
Next, we investigated the high-temperature-resistant capability in vacuum of our devices. Since mica substrate cannot endure temperature above 750°C, we replaced it with Si substrate with 300 nm SiO 2 on top. We heated the WSe 2 FETs at 1000°C for 15 min in vacuum with Argon flow rate of 100 sccm. The WSe 2 FET showed negligible change after heating (Supplementary Fig. 7). Raman spectrum demonstrates prominent WSe 2 character peaks and no WO 3 character peak, indicating that WSe 2 lattice structure remained intact after heating. As a control group, bare WSe 2 flake on SiO 2 /Si substrate completely vanished after 1000°C heating ( Supplementary Fig. 7). Therefore, the h-BN/GF structure effectively protects WSe 2 at ultrahigh temperature of 1000°C in vacuum. Dark current increased slightly after 700°C in air and 1000°C in vacuum annealing ( Supplementary Fig. 8). The temperature that our devices can tolerate is much higher than that of current 2D material devices, both in air and vacuum environments (Table 1). Our research expands the working temperature range of 2D materials, allowing the good electrical properties of 2D materials to be applied in high-temperature environments.
Graphite flake electrodes are essential to high-temperature protection. We replaced the GF in WSe 2 FET with conventional (Pt) metal electrodes (Fig. 2d). After heating at 500°C for 10 min, WSe 2 flake within different sizes of h-BN encapsulation was strongly oxidized (Fig. 2e, Supplementary Fig. 9 and Supplementary Table 1). This is because Pt film deposited by sputtering has much larger surface roughness than that of GF. Top h-BN cannot completely conform to the topography of Pt surface (HRTEM crosssectional image in Fig. 2g), leading to oxygen molecules diffusion into h-BN encapsulation through Pt/h-BN interface. Atomic force microscopy (AFM) characterization demonstrates that the height variation of Pt surface is~6.8 nm ( Supplementary Fig. 10), while the height variation of GF measured (~0.6 nm) appears to be limited by instrument noise and is identical to that obtained from the surface of highly oriented pyrolytic graphite (HOPG) which approaches the limit of atomic flatness. As such, good contact is formed between h-BN and GF (HRTEM cross-sectional image in Fig. 2f) which can effectively prevent oxygen diffusion. To further prove the oxygen diffusion through Pt/h-BN interface, we transferred a WSe 2 flake on Pt surface and covered it with h-BN. After heating at 500°C for 15 min, WSe 2 inside h-BN/Pt encapsulation was strongly oxidized ( Supplementary Fig. 11). GF/WSe 2 /h-BN sandwich structure was prepared and tested under the same experimental condition. The WSe 2 flake inside h-BN/GF encapsulation was still in good shape after heating. Therefore, GF electrode prevents oxygen diffusion and plays an important role in high-temperature protection.

High-temperature electrical properties
We systematically investigated the temperature-dependent electrical properties of the WSe 2 FET in dark environment without the influence of photoexcitation. Electrical measurements were carried out below 550°C for safety reason. Figure 3a demonstrates the transfer curves of a representative WSe 2 device with channel width/ length of 10 μm/5 μm (source/drain voltage V ds = 100 mV). At room temperature (20°C), on/off ratio of 2 × 10 6 and subthreshold swing (SS) of 130 mV/dec were obtained (SS = dV gs /dlgI ds , where V gs is gate bias, and I ds is source/drain current). Carrier mobility derived from p branch reached~35 cm 2 /V·s. The excellent room-temperature electrical performance is comparable to that of the best WSe 2 field-effect transistors reported 28,29 . As the temperature increased, larger I ds for all values of gate voltage from −3 V to 3 V were observed, and the device demonstrated ambipolar behavior under different temperatures (Fig. 3a). Under 500°C, the WSe 2 FET still showed good transfer properties. Linear and symmetric I ds -V ds curves obtained at 500°C further demonstrated ambipolar behavior and suggested near-ohmic contact between WSe 2 and GF electrodes (Fig. 3b). Photoluminescence spectra illustrated that the bandgap of WSe 2 decreased from 1.55 eV to 1.40 eV as temperature increased from 150°C to 500°C (Supplementary Fig. 12). Theoretically, smaller bandgap resulted in smaller on/off ratio. As temperature varied from 20°C to 500°C, on/off ratio of the WSe 2 FET decreased from 2 × 10 6 to 1 × 10 2 (Fig. 3c). According to the where k is Boltzmann constant, T is absolute temperature, q is the charge per carrier, C ox and C s are dielectric capacity and depletion capacity, respectively, higher temperature leads to larger SS. The SS of our device increased from 130 mV/dec to 600 mV/dec as the temperature varied from 20°C to 500°C (Fig. 3c). After 500°C heating in air, the device was tested at room temperature again. Interestingly, due to the high temperature (500°C) annealing, which improves WSe 2 /GF contact, the electrical properties of our device did not degrade but slightly improved (SS became smaller, Supplementary Fig. 13). Lower resistance of GF electrode was observed after high temperature annealing ( Supplementary Fig. 14), indicating that annealing will not affect the conducting properties of GF electrode.
The off-state current I off of WSe 2 FET increased from 10 −12 A to 10 −8 A as temperature varied from 20°C to 500°C (Fig. 3d). The h-BN gate dielectric leakage current (I gs ) of the same device measured at the same V gs and V ds under the same temperature was 1-2 orders of magnitude smaller than I off (Fig. 3d), indicating that the off-state current of WSe 2 FET at high temperature is dominated by the intrinsic turn-off characteristics of WSe 2 instead of h-BN leakage current. Therefore, h-BN is not only a good oxygen-resistant coating, but also an effective high-temperature dielectric layer. The remarkable high-temperature isolation properties of h-BN contribute to the high-performance of our WSe 2 devices.

WSe 2 photodetector with negative photoconductivity
We next explored the photoelectric characteristics of the WSe 2 photodetectors. Under 25 W/m 2 white light illumination at 20°C, the I ds -V gs transfer curve moved upward for all values of gate bias from −3 V to 3 V (Fig. 4b), indicating positive photoconductivity (PPC). Interestingly, under high temperature (400°C), white light illumination resulted in a left shift of transfer curve (Fig. 4c). Larger shift was observed as light intensity increased. The N branch of transfer curve mainly demonstrated PPC, while P branch demonstrated negative photoconductivity (NPC). NPC phenomenon has been observed in low dimensional materials at room temperature [31][32][33] . As V gs was set at a constant value of 3 V, I ds increased under white light illumination at 400°C, and the WSe 2 device act as a PPC photodetector (Fig. 4d). In contrast, as V gs was set at 0 V, I ds decreased under illumination at 400°C, and the WSe 2 device act as an NPC photodetector (Fig. 4e). Therefore, hightemperature reconfigurable photodetector was realized which can switch between NPC and PPC photodetector under the same temperature by adjusting gate voltage V gs . NPC and PPC photodetector are building blocks of photoelectric logic gate. Reconfigurability makes it a great advantage for our device to be applied to photoelectric logic gate.
To investigate the origin of the unconventional NPC phenomenon, control groups were prepared and tested under the same experimental condition: 1) WSe 2 with GF electrodes and mica encapsulation (two mica flakes with the thickness of 40-60 nm were placed on top and bottom of WSe 2 ), 2) Bare WSe 2 with GF electrodes (without h-BN encapsulation), 3) Bare WSe 2 with Pt electrodes (without h-BN encapsulation), 4) WSe 2 with Pt electrodes and h-BN encapsulation. The high-temperature measurements were done in a short time to minimize the oxidation of WSe 2 . V gs =0 V, or V gs was not applied. The first 3 types of devices all demonstrated PPC from room temperature to 400°C (Supplementary Figs. 15-17). Only the last type of devices demonstrated NPC at 400°C (Supplementary Fig. 18), indicating that h-BN is responsible for the NPC phenomenon.
At relatively low temperature, photoexcited electron-hole pairs are restricted in WSe 2 channel due to the excellent insulation of h-BN, and can be extracted by applying V ds . As such, the current increases after photoexcitation (process 1 in Fig. 4f). The h-BN gate dielectric leakage current increased from 10 −13 A to 10 −10 A as temperature varied from 20°C to 500°C (Fig. 3d), implying that the chance for charge carriers to enter h-BN layer significantly increases at elevated temperature. Photogenerated holes trapped by the defect states inside h-BN act as an equivalent positive gate voltage (photogating effect, process 2 in Fig. 4f), leading to a left shift of I ds -V gs transfer curve as shown in Fig. 4c and Supplementary Fig. 19. Photogenerated charge carriers trapped by the defect state inside low-dimensional materials is one of the main reasons for NPC phenomenon 34 . For example, Xu et al., reported room temperature NPC phenomenon of graphene/black phosphorus heterojunction which is due to the electrons trapped in black phosphorus 35 .
To figure out the temperature that process 2 starts to dominate and the impact of light wavelength, we utilized 385 nm wavelength purple light (18 W/m 2 ), 440 nm wavelength blue light (480 W/m 2 ), and 532 nm wavelength green light (440 W/m 2 ) to illuminate the WSe 2 photodetector respectively (V gs = 0 V) under different temperatures. The photodetector exhibited PPC at low temperatures (Fig. 5a). For all the three wavelengths, the device switched to NPC as temperature reached~275°C, indicating that NPC phenomenon happens in a wide wavelength range. It is worth mentioning that the NPC occurs due to the defect states inside h-BN, which means that different WSe 2 photodetectors may exhibit slightly different PPC/NPC transition temperatures. The absolute value of NPC responsivity was significantly larger than that of PPC at room temperature (under 385 nm illumination, the responsivity at 400°C was~2000-fold higher than that at room temperature), and shorter wavelengths usually resulted in higher responsivity at the same temperature. Our devices exhibited ultrahigh responsivity of 2.2 × 10 6 A/W at 400°C under 0.2 W/m 2 365 nm illumination ( Fig. 5b and Fig. S17). At 500°C, we also obtained an impressive photoresponsivity of 1.1 × 10 6 A/W ( Supplementary Fig. 20). The photoresponsivity is not only higher than that of state-of-the-art high-temperature photodetectors, but also higher than that of existing WSe 2 devices (Fig. 5d). High responsivity at high temperatures results in a large signal-to-noise ratio of 152.9, which is sufficient to meet the needs of optical sensing (Supplementary Fig. 21 and Supplementary Table 2). The detectivity D* can be derived from 12 : where A is the sensing area, R is the photoresponsivity, q is the unit of charge and I d is the dark drain current. Our device demonstrate detectivity of 1.6 × 10 13 Jones at 400°C and 2.63 × 10 12 Jones at 500°C. The detectivity of our devices is comparable to those of state-of-the-art 2D materials, III-V materials, and Si photodetectors (Supplementary Table 3).

Responsivity(A/W)
Temperature( o C) of strain (~0.17%) and high temperature. The dynamic resistance variation is shown in Fig. 5c. The resistance of the device increased rapidly from 1.5 MΩ to 2.5 MΩ within 0.3 s after illumination, indicating NPC phenomenon. As the light was turned off, it took approximately 13 s for the resistance to dropped back to the dark state value. This process was repeated for over 5 times and demonstrated good repeatability. Therefore, our flexible photodetectors can adapt to non-coplanar working conditions with good optical sensing performance which cannot be achieved by conventional rigid high-temperature photodetectors.

Discussion
We developed WSe 2 photodetectors which can endure temperature up to 700°C in air and 1000°C in vacuum. Our research greatly expands the working temperature range of 2D materials, allowing the good properties of 2D materials to be applied in high temperature environments. The device exhibited unconventional NPC phenomenon, and the photoresponsivity in NPC mode (2.2 × 10 6 A/W) is~10 5 -fold higher than that of existing high-temperature photodetectors, and also higher than that of existing WSe 2 photodetectors. This work bridges the technology gap between highly-sensitive photodetectors and hightemperature photodetectors. Current high-temperature photodetectors are rigid, which largely limits their applications. Our device is both highly flexible and can endure ultrahigh temperature and thus we realized in situ high-temperature optical sensing under bending state. Our approach opens up opportunities for 2D-material devices working in harsh environment, and may stimulate fundamental research of the fascinating properties and new phenomena of 2D materials at high temperature.

Device fabrication
A freshly cleaved mica substrate was firstly immersed in acetone, alcohol, and deionized water successively for 2 min ultrasonic cleaning in order to remove possible impurities. Then, bottom h-BN (40-60 nm), WSe 2 (10-20 nm), two GF electrodes and top h-BN (40-60 nm) were mechanically exfoliated using scotch tape and transferred onto mica substrate by PDMS film. The transfer process was carried out using an accurate transfer platform (Metatest, E1-T). Finally, a platinum electrode of 30 nm thick as top gate was fabricated using photolithography, metal deposition, and a lift-off process. Asfabricated devices were annealed in N 2 atmosphere at 250°C for 30 min to remove residuals between the layers.

Device characterizations
The thicknesses of the 2D materials and the surface roughness of GF and Pt were characterized by AFM (Bruker, Dimension Icon) using ScanAsyst-air mode. The Raman spectra were derived by Raman spectrometer (HORIBA Jobin Yvon, LabRAM HR Evolution) with 532 nm laser (0.325 mW/cm 2 ) and acquisition time of 120 s. The crosssectional images of van der Waals heterostructures and h-BN/Pt interface were characterized by high-resolution transmission electron microscopy (JEOL-2100 TEM) with acceleration voltage of 200 kV. The samples for HRTEM were prepared using Xenon focused ion beam (Helios G4). High-temperature photoluminescence (PL) spectrum was detected on a temperature-control stage (Inspec HCP621G+) by PL spectrometer (Andor KYMERA-328I-B1, grating 300 grooves/mm), and a 532 nm continuous wave laser (commercial Coherent Genesis MX, intensity = 0.05 mW) was applied.

Device measurements
The transfer characteristics and I-t characteristics of the WSe 2 FETs were measured by semiconductor parameter analyzer (Agilent B1500A). The devices were fixed and tested on a temperature-control stage. For the purpose of investigating electrical characteristics of the devices under strain, we fixed it on the home-made matrix with different bending radius, and then tested it with semiconductor parameter analyzer. The dynamic resistance variation of the photodetectors was measurement by digital multimeter (Agilent 34470A). The devices were fixed and tested on a ceramic heating rod.

Data availability
Relevant data supporting the key findings of this study are available within the article and the Supplementary Information file. All raw data generated during the current study are available from the corresponding authors upon request. Source data are provided with this paper.
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